The objective of the project is to determine if the growth and development of cancer cells can be influenced by contact with cells of the embryo. The mouse will be used as the experimental animal. Cells from (1) Testicular teratomas, (2) Tumors adapted to subcutaneous transplantation, (3) Cell culture lines derived from tumors, and (4) Spontaneously occurring tumors will be introduced separately into the blastocoele cavity of the preimplantation mouse embryo. The effect on the cells will be ascertained (1) By microscopic changes in the cells during in vitro development of the blastocyst, (2) Autoradiographically following the fate of the cells in the embryos, and (3) By placing the recipient blastocysts into foster mothers and studying the offspring for signs of an effect from the transferred cancers cells. Some cancer cells may have the ability to clone the blastocyst and participate in embryo development.